In various audio processing apparatuses which process digital audio signals (called as “audio data”) such as a digital mixer, it has been conventionally performed that the audio data is inputted from external device. Transmission of the audio data in this case may be performed through methods (protocols) using audio transmission lines such as AES-3 ™, MADI™ and ADAT™, and methods (protocols) using an audio network such as CobraNet™, Dante™ and EtherSound™, for example.
Here, it is common to the above-stated various transmission methods that a word clock (timing information) indicating timing (reproduction timing) to reproduce each sample of the audio data at a digital analog converter (DAC) is carried with the audio data, though formats of the timing information clock and procedures for the carrying are different depending on each method. Note that substances of the word clocks are various depending on the methods. In some method, the reproduction timing of samples is indicated by a change timing of a voltage of an electrical signal. In another method, the reproduction timing of samples is indicated by a time stamp. This word clock (in a broad sense) is a broader concept than a “word clock in a narrow sense” of one clock per one sample. In the present description, when it is stated as the “word clock” without distinguishing between the narrow sense and the broad sense, the “word clock in the broad sense” is indicated.
For example, in the audio transmission line, a reception timing (word clock) itself when each sample of audio data reaches a reception side is a reproduction timing of the sample. Since other data do not flow in a cable of the audio transmission line, a reception cycle of each sample, namely, a cycle of the word clock is stable.
Besides, according to EtherSound™, a reception side apparatus periodically (synchronous periodically) receives a packet in each cycle synchronous with reproduction timing of samples of audio data, and in each packet, there is one sample of audio data for each of a plurality of channels. A reception timing (word clock) when the packet reaches the reception side corresponds to the reproduction timing of each sample in the packet.
According to CobraNet™, a reception side apparatus synchronous periodically receives packets, and in each packet, there are continuous “n” samples of audio data for each of a plurality of channels. A reception timing (word clock) when the packet reaches the reception side corresponds to a reproduction timing of “n” samples, and the reproduction timing of each sample is obtained by 1/n subdividing each cycle of the reception timings (in other words, multiplying a frequency of the packets by n).
Note that according to such audio networks, packets other than the audio data also flow in the cable, and therefore, there is a slight fluctuation in a synchronous periodical transmission at a transmission side apparatus, and the reception timing of the packet, namely, the word clock becomes unstable compared to the audio transmission line.
According to Dante™, a reception side apparatus periodically (asynchronous periodically) receives a packet in each cycle asynchronous with reproduction timing of samples of audio data, and in each packet, there are a continuous plurality of samples of audio data for each of a plurality of channels and a time stamp (word clocks) of every n samples. Accordingly, the timing indicated by each time stamp is a reproduction timing of “n” samples of the audio data, and the reproduction timing of each sample is obtained by 1/n subdividing each cycle of the timings indicated by the time stamps (in other words, multiplying the frequency of the time stamps by n).
Note that according to such an audio network, the reproduction timing of each sample generated based on the time stamp is usually unstable compared to the case of the audio transmission line because there is a lag in time (a reference unit of the time stamp) commonly used among a plurality of apparatuses and there are delay and missing of the packets.
Incidentally, when the received audio data is reproduced at the reception side apparatus, a clock with very high frequency-stability is required as a sampling clock which defines the reproduction timing so as to enable a high-quality output. Such frequency-stable sampling clock can be generated by, for example, inputting the above-stated word clock to a master PLL (phase looked loop: phase synchronization) circuit whose cutoff frequency is extremely low.
However, when the audio data is received through a plurality of audio transmission lines and audio networks, the reception side apparatus also acquires the word clocks via each transmission line or via each network.
In such a case, one of the acquired word clocks is selected in accordance with an operation of a user or automatically, and the sampling clock is generated by inputting the selected word clock to the master PLL circuit, in a conventional apparatus described in PTL1 or PTL2.